I beg to differ - there are times when it becomes obvious someone is looking for an opening and a warning shot may change their mind and negate the need to kill them.
Rule 1: Treat every weapon as if it were loaded. (2) Rule 2: Never point a weapon at anything you do not intend to shoot. (3) Rule 3: Keep your finger straight and off the trigger, until you are ready to fire. (4) Rule 4: Keep the weapon on safe, until you intend to fire.